Masks for supplying a desired atmosphere to the wearer, such as oxygen masks and masks for removing dust and pollen, are well known and widely used. A problem is that they seldom fit the wearer properly. This is because such masks typically are provided in only one size. Such masks may fit an adult of average size reasonably well, but they fit poorly on a larger or smaller adult. At best, such a mask is uncomfortable; at worst, its effectiveness is diminished at least somewhat.